The invention relates to methods of and apparatus for detecting radionuclides in boreholes, groundwater, effluent streams, soil and other scenarios. In particular aspects, the invention pertains to methods of and apparatus for discriminating between high energy particles such as cosmic rays or natural terrestrial radiation having energies greater than an upper threshold, and lower-energy radiation from decay of specific nuclides.
In conjunction with operation of manufacturing facilities for production of medical and other radioactive materials, operation of nuclear power and breeder facilities and storage of waste products from all of these kinds of facilities, it is desirable to be able to determine the presence and concentration of certain radionuclides and to be able to discriminate between radiation from different kinds of radionuclides as well as high energy cosmic rays or natural terrestrial radiation. It is of particular interest to be able to selectively probe boreholes drilled in the vicinity of these types of facilities in order to detect presence of or track movement of these kinds of radionuclides.
Various kinds of measurement instruments and techniques have been developed for these types of assays. One conventional approach involves extracting samples of material, such as soil or groundwater, and then shipping these samples to laboratories for traditional analysis using radiochemical techniques. In the case of Sr-90, this is typically carried out using chemical separation followed by liquid scintillation counting. Disadvantages of this approach include expenses and delay between sample extraction and the availability of results. Additionally, the repeated handling of the samples by different personnel in different locations increases the possibility of error in attribution of test results to a specific test site.
Another approach relies on scintillation counting using photomultiplier tubes coupled to specialized plastic scintillation media. An instrument developed by Alan Schilk of Pacific Northwest Laboratories shows a sensitivity of about 4 picoCuries per gram of sample. Disadvantages to this approach include high instrument costs, instrument size, need for multiple photomultiplier tubes and sensitivity to environmental variables such as temperature.
It would be desirable to develop alternative methods for measurement of radioactivity, especially that due to Sr-90 and U-238. It would be particularly desirable to develop alternative and portable methods and apparatus for in situ, real-time characterization of migration of materials from buried nuclear waste, in nuclear facility effluent and within existing structures providing constricted access.